1. Field of the Invention
This invention relates to a novel method and apparatus for applying cathodic protection to tubular members such as pipelines and flow lines used in the oil and gas industry. The invention is particularly suited to pipelines and flow lines in offshore applications using coiled tubing as the cathodic protection can be applied as the coiled tubing is deployed from a lay vessel. These pipelines and flow lines are usually buried underground whether on land or on the ocean bottom in a trench and the trench filled in with soil. The pipelines and flow lines are then subject to corrosion because of the chemical reactions occurring between the soil constituent elements and the metal pipe.
In order to protect the metal pipeline or flow line from this sort of cathodic or galvanic cell corrosion, the pipelines and flow lines have traditionally been protected in one of several ways. The first method used is to protect or insulate the pipe from the surrounding soil. This is done by coating the exterior of the pipe to prevent any galvanic or corrosion reaction occurring. Typical methods for this type of protection have included fusion-bonded epoxy coatings, enamel coatings or tape coating. Fusion-bonded epoxy coatings are best suited to applications where the pipe can be coated at the mill due to the high temperatures required to apply the coating.
Conversely, enamel and tape coatings usually are applied as the pipeline or flow line is being laid. Enamel coatings such as coal tar enamel are heated and applied to the pipe's exterior and then the pipe is wrapped with fiberglass, felt and kraft paper to protect the coal tar enamel while it hardens. Tape coatings are usually applied in one continuous operation by a single machine with the pipe being cleaned, coated and wrapped with a tape such as polyethylene or polyvinyl in a spiral manner with overlapping segments. All three of these methods suffer from the fundamental problem of any damage to the coating during the pipe laying process leaves the underlying pipe exposed and extremely vulnerable to corrosion and therefore structurally weak spots can form leaving the pressure containing capacity of the pipeline or flow line compromised.
Another method used is to leave the pipeline or flow line uncoated and attach sacrificial anodes, usually of zinc, to the pipe at predetermined spacings. This method also can be used as an additional measure with the protective coating type methods described above. This method often is only partially successful as the sacrificial anodes are damaged during the laying process, particularly in offshore settings, thereby leaving the pipe essentially unprotected. Additionally, if the sacrificial anode is used with the protective coating methods, the sacrificial anodes are spaced further apart than if the pipe is uncoated. This leads to problems when the protective coating is damaged at a point distant from the sacrificial anodes. Corrosion can begin at the damaged point and can move along the pipe under the protective coating, undetectable by visual inspection.
The present invention minimizes these various shortcomings and hazards by providing a method and apparatus to apply a corrosion protection to pipelines and flow lines that are both a protective coating and a cathodic protection. Recent advances in continuous casting technology allow a tape of a sacrificial anodic material, such as zinc or aluminum, to be formed with the tape's width and thickness held to a close tolerance. Thus, the zinc tape can be manufactured with a thickness of 10 mils or a thickness of 100 mils. An appropriate thickness can then be chosen depending on the degree of protection required, i.e., the expected length of time the pipeline will be in service. This advance along with advances in metallurgical technology that allow a high degree of purity in the sacrificial anode material make the current invention feasible. The current invention also offers advantages in leak detection of the underlying tubular member. The prior methods have the problem of allowing corrosion to form under the protective coating, undetected. The current invention helps to eliminate this problem as its thin coating will not contain a leak and therefore it is easily detectable.
A final problem with current technology relates to the fact that more and more oil and gas exploration and discovery is being done in deeper offshore locations. This means the wells are in more remote locations which means the pipelines and flow lines connecting these wells to the current infrastructure must be longer. These longer pipelines and flow lines in deeper water require more anodes because the colder water at the deeper slows the reaction of the sacrificial anodes, i.e., reduces the cathodic "throwing power" of the sacrificial anode as it is referred in the industry. The current invention overcomes these problems by providing a metallurgically bonded continuous sacrificial anode material on the pipeline or flow line. An additional benefit of the current invention is the sacrificial anode material tape of the current invention is much more compact than the previously described coal tar enamel or epoxy coatings. The smaller diameter of the coated tubular member using the current invention means more pipeline or flow line can be carried on the lay barge.
2. Description of Related Art
A number of patents have been issued for various machines to coat and wrap pipe and thereby leave a protective barrier.
One such machine is shown in U.S. Pat. No. 2,070,925 to E. L. Rolfs et al. that wraps tape onto a previously asphalt coated pipe in an overlapping manner.
U.S. Pat. No. 2,770,284 to E. L. Myrick discloses another machine that applies a heated coating material, wraps the pipe with a paper or like material, and transports itself along the pipe.
Another machine that wraps a fiber glass, kraft paper or plastic film tape onto a previously applied enamel coating is disclosed in U.S. Pat. No. 3,190,780 to F. E. McNulty etal.
U.S. Pat. No. 3,990,478 to W. H. McFarland discloses wrapping a pipe with a high tensile strength steel strip that is coated with a less noble metal, particularly zinc. The steel strip wrapping is used to strengthen the pipe. The zinc coating of the strip is electrically connected to the pipe being protected to complete the galvanic protection. Additionally, an embodiment is shown in which an external DC power source is used to augment the zinc coated strip.
An adhesive backed tape with galvanic sacrificial anode metal foil pieces is disclosed in U.S. Pat. No. 4,645,697 to Y. Torigoe. The tape is a vinyl material such as polyethylene or polyvinyl chloride and is used to wrap the pipe with the anode metal foil pieces contacting the pipe surface.